Plant growth is a highly malleable process that is strongly influenced by environmental factors, especially light. Light plays a vital role in plants' photomorphogenesis and affects almost all aspects of plant growth and development. The effects of light on plant development are especially prominent at the seedling stage. Under normal light conditions with unobstructed direct light, a plant seedling develops according to a characteristic photomorphogenic pattern, that is, it has open, expanded cotyledons and a short hypocotyl. This developmental pattern rapidly establishes the seedling as a photoautotrophic organism, and most of the plant's energy is devoted to cotyledon and leaf development while longitudinal extension growth is minimized. A seedling growing in darkness, however, will etiolate, displaying elongated hypocotyls and closed and unexpanded cotyledons. Under low light conditions where light quality and intensity are reduced by shading, obstruction or high population density, a seedling develops according to a different pattern as a shade-avoiding seedling that displays reduced cotyledon expansion relative to the seedling grown in unobstructed light, and hypocotyl extension is greatly increased. During this developmental response of the seedling to the low light conditions, the hypocotyl is elongated which couples with reduction in cotyledon and leaf expansion.
Thus, a significant problem for crop farming is created when crop plants are grown at high population density as it often results in a low light level for each individual plant. To compete for this light, plants have to re-distribute their energy and nutrition towards height extension, often called a shade avoidance response, resulting in an accelerated stem elongation and thin stems. This shade avoidance response to poor light conditions in a populated environment often results in crop yield loss. For example, in maize plants, accumulating evidence suggests that the stem elongation process itself may be linked to suppression of ear development. Corn prolificacy and ear establishment are sensitive to light intensity. High population density may cause abortion of ear development at lower nodes, even at all nodes. High density leads to most of the red and blue spectra of the sunlight being absorbed by the upper leaves, leaving the far-red light filtered or reflected to the lower canopy. The red/far-red ratio is a function of canopy density. If the density is high, the red/far-red ratio is low. This low ratio triggers the shade avoidance response, in which the plants distribute resources for stem elongation in a competition for sunlight (Quail et al, Science 268, 675–680, 1995). Reduction or elimination of the shade avoidance response has been shown to improve harvest index or yield (Maliakal et al, Critic. Rev. Plant Sci. 17, 465–539, 1999; Thiele et al, Plant Physiol. 120, 73–81, 1999; Robson et al, Nature Tech. 14, 995–998, 1996). Thus, the shade avoidance response is relevant to the harvest index, for example at high population density.
Various attempts have been made to overcome the shade avoidance problem in crop farming. Breeding efforts usually result in shorter plants and, in the case of corn, smaller tassels to save energy and nutrition for kernel development (Duvick and Cassman, Crop Sci. 39, 1622–1630, 1999; Chapman and Edmeades, Crop Sci. 39, 1315–1324, 1999). Molecular and biotechnological approaches have also been tried to identify a gene or a set of genes that manipulate the photomorphogenesis pathway in a manner modifying the plant architecture to have shorter internodes. Such a plant, when growing in a dense population, would have the ability to respond to low light environment without extending its stem, thereby minimizing the shade avoidance response and enhancing yield (see, for example, Smith, U.S. Pat. No. 5,945,579; Hershey and Keller, U.S. Pat. No. 5,268,526; Deng et al., PCT Application WO00/18940).
In recent decades, many genes or gene mutants in light-signal transduction and shade avoidance response pathways have been identified and studied (Chory, Plant Cell 9: 1225–1234, 1997; Chory et al., Cell 58: 991–999, 1989; Deng et al., Genes Dev. 5: 1172–1182, 1991; Karlin-Neumann et al., Plant Physiol. 88: 1323–1331, 1988; Lissemore and Quail, Mol. Cell. Biol. 8: 4840–4850, 1988; U.S. Pat. No. 5,945,579; McNellis and Deng, Plant Cell 7: 1749–1761, 1995; Nagatani et al., Plant Physiol. 102: 269–277, 1993; Osterlund et al., Trends Cell Bio. 9: 113–118, 1999; Parks and Quail, Plant Cell 5: 39–48, 1993). Among these genes, a constitutive photomorphogenesis 1 gene (COP1) from Arabidopsis has been studied and demonstrated to be regulated by light during plant development in response to different light conditions (Osterlund et al., Trends Cell Bio. 9: 113–118, 1999; Deng et al., Cell 71: 791–801, 1992; McNellis et al., Plant Cell 6: 1391–1400, 1995; McNellis et al., Plant Cell 8: 1491–1503, 1996; Osterlund and Deng, Plant Journal 16 (2): 201–208, 1998; Stacey et al., Plant Cell 11: 349–363, 1999; Torii et al., EMBO 17: 5577–5587, 1998; von Arnim and Deng, Cell 79: 1035–1045, 1998; Yamamoto et al., Plant Cell 10: 1083–1094, 1998; Deng et al., PCT Application WO01/18940). The COP1 gene was initially identified through recessive loss-of-function mutations in Arabidopsis that display a constitutively photomorphogenic phenotype regardless of light conditions (Deng et al., Genes Dev. 5: 1172–1182, 1991). The constitutively photomorphogenic phenotype and recessive nature of cop1 mutations indicate that COP1 may act as a negative regulator, or light-inactivated repressor, of photomorphogenesis. The COP1 gene in Arabidopsis encodes a protein that contains three recognizable domains: a ring finger domain (zinc-binding motif), a coiled-coil domain and multiple WD-40 repeats characteristic of the B subunit of trimeric G-proteins (Deng et al., Cell 71: 791–801, 1992; PCT Application WO00/18940). These protein domains have been implicated in protein—protein interactions, and thus COP1 might interact with multiple partners via these interactive domains to regulate plant morphogenic development and the shade avoidance response. Overexpression of a full-length COP1 results in quantitative hypersuppression of photomorphogenic development (McNellis et al., Plant Cell 6: 1391–1400, 1995), which suggests that COP1 plays a role in a regulatory step in mediating the repression of photomorphogenic development (Osterlund et al., Trends Cell Bio. 9: 113–118, 1999; Deng et al., PCT Application WO 00/18940). The wild-type COP1 protein normally acts to repress the photomorphogenic pathway in darkness and light reverses this repression. COP1 appears to be a downstream light-signaling component (Deng et al., Cell 71: 791–801, 1992; PCT Application WO 00/18940). Overexpression of a fragment of COP1 in Arabidopsis is hypothesized to down regulate native COP1, this has also resulted in shorter stems of transgenic plants growing under low light conditions in comparison with those of wild-type plants (see, Deng et al., PCT Application WO 00/18940).
Thus, the COP1 proteins in plants growing at low light conditions such as in a highly populated environment will act to repress normal photomorphogenic development of these plants and help activate shade avoidance response pathway to stimulate stem elongation. Therefore, reducing the level of functional COP1 proteins in plants might produce a phenotype typical of plants growing at high light intensity conditions even when the plants are under low light conditions. This phenotype could include well developed leaves, more chloroplasts, shorter and thicker stems.
Although some studies have been done to understand the role of COP1 proteins in plant morphogenesis and development, there is little reported effort on utilizing COP1 to deal with an unsolved, common problem in crop farming; that is the shade avoidance response of plants. Deng and his colleagues (Deng et al, PCT application WO 00/18940) disclosed an isolated COP1 nucleic acid from Arabidopsis and use of said COP1. Their publication was directed to improved seedling emergence characteristics and not to a solution to shade avoidance related problems in crop plants grown at high population density.
Thus, there exists a need in the field for a new and different approach to reduce or diminish the shade avoiding response of crop plants growing at high population density. There exists a need, through use of a different light transduction component, i.e., COP1 gene, to improve some of crop plants' agronomic traits such as reduced stem length and increased shade tolerance that are closely associated with crop yield.